It is well known that systems such as heat pumps or the like operate on a reverse cycle principle. In general, such a system includes indoor and outdoor heat exchange coils which are exposed to their respective ambient conditions. When the temperature of the indoor air is to be raised, the compressor pumps the volatile refrigerant fluid through the indoor coil and subsequently, by way of expansion means, through the outdoor coil of the system. The latter then acts as an evaporator by absorbing heat from the outdoor air. Simultaneously, the indoor coil acts as a condenser which gives off the heat previously absorbed from the outdoor air to the indoor environment.
Since the outdoor coil under these conditions operates at a temperature less than the ambient outdoor air, moisture carried by the latter condense on the outdoor coil and causes frost to be built up thereon. This layer of frost acts as an insulator between the outdoor air and the outdoor coil and so prevents effective heat transfer to the latter. Under these conditions, system efficiency is greatly reduced. Further, the frost on the coil may encase whatever sensing element is positioned thereon so as to produce a false reading of the condition which is intended to be sensed.
A similar action occurs with respect to the indoor coil when the latter is used as an evaporator in order to cool the temperature of the indoor air. Moisture carried by the indoor air is deposited on the indoor coil in the form of frost and reduces the efficiency of the system.
Workers in the air conditioning field are well aware of the problem of controlling the build-up of frost in temperature conditioning systems of this type and numerous schemes have been advanced to deal with it. It is recognized that the defrosting action itself may be carried out in a number of ways, e.g. by reversing the operation of the system, by terminating the system operation, by blowing heated air across the frost-carrying coil, etc., or by a combination of some or all these methods. The difficulty arises primarily in automatically choosing the points at which the defrosting action is to be initiated and terminated respectively, so as to maintain system operation at maximum efficiency. The problem is compounded by the fact that these operating points change with changing ambient conditions. Thus, while at low ambient outdoor temperatures it takes longer to accumulate a given thickness of frost than at higher temperatures, the defrosting action will also take longer than when a higher outdoor temperature prevails. Thus, the points at which the defrosting action of the outdoor coil are initiated and terminated respectively, must vary as a function of the outdoor air temperature in order for the system to operate at maximum efficiency.
Other ambient conditions will also affect the amount of frost which collects on the outdoor coil. For example, the humidity of the ambient air will determine in some measure the rate of frost build-up. Further, precipitation such as snow or freezing rain will affect the amount of frost deposited on the outdoor coil.
An effective frost control system must be capable of taking into account all of these ambient outdoor conditions in order to keep the outdoor coil frost-free and so as to operate the temperature conditioning system at high efficiency.
The deposit of frost on the indoor coil is affected only by some of the factors discussed above. These are primarily the temperature of the ambient indoor air and its humidity. An effective frost control system must also be capable of dealing with the latter conditions in order to provide efficient operation throughout.
A further requirement of an effective frost control system is reliability of operation under different and adverse conditions. Since the system is ordinarily serviced only at infrequent intervals, it must be capable of maintaining efficient operation for long time intervals without human intervention. Such a requirement dictates a reduction of the number of parts ordinarily found in prior art frost control apparatus which are subject to breakdown or to malfunction, as well as the ability to stand up under varying, adverse operating conditions.
Finally, the cost contribution of the frost control apparatus to the overall cost of the temperature conditioning system of which it is a part must be considered. This includes the initial cost, as well as the cost contribution to the maintenance of the overall system. The latter may be considerable if the frost control apparatus is complex and subject to break-down.
Earlier attemps at solving the problems associated with effective frost control have not been successful in all the areas discussed above. Much of the proposed equipment, particularly that which employs relatively complex mechanical apparatus to achieve frost control, is expensive to build, difficult to maintain and it is prone to break down or malfunction if left unattended for extended periods of time. Moreover, such equipment is difficult to adjust accurately for optimum operation and to maintain in adjustment during operation. As a consequence, optimum operating efficiency is rarely, if ever, achieved with such equipment, while the associated costs are often high. Still other prior art frost control equipment operates strictly on the basis of temperature check points. The operation of this type of equipment is frequently inaccurate and erratic since it fails to take into account the varying relationships of the relevant operating factors as the ambient conditions change.